Acid-soluble cement compositions comprising cement kiln dust and methods of use

ABSTRACT

The present invention relates to acid-soluble cement compositions that comprise cement kiln dust (“CKD”) and associated methods of use. An embodiment provides a method of cementing comprising: providing an acid-soluble cement composition comprising a kiln dust and water; allowing the acid-soluble cement composition to set to form an acid-soluble hardened mass; and contacting the acid-soluble hardened mass with an acid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/825,004, entitled “Acid-Soluble Cement CompositionsComprising Cement Kiln Dust and/or a Natural Pozzolan and Methods ofUse,” filed on Jun. 28, 2010, which is a continuation-in-part of U.S.patent application Ser. No. 12/606,381, issued as U.S. Pat. No.7,743,828, entitled “Methods of Cementing Subterranean. FormationFormations Using Cement-Kiln Dust in Compositions Having ReducedPortland Cement Content” filed on Oct. 27, 2009, which is acontinuation-in-part, of U.S. application Ser. No. 12/420,630, issued asU.S. Pat. No. 7,631,692, entitled “Settable Compositions Comprising aNatural Pozzolan and Associated Methods,” filed on Apr. 8, 2009, whichis a continuation-in-part of U.S. patent application Ser. No.12/349,676, issued as U.S. Pat. No. 7,674,332, entitled “ExtendedSellable Compositions Comprising Cement Kiln Dust and AssociatedMethods,” filed on Jan. 7, 2009, which is a divisional of U.S. patentapplication Ser. No. 12/034,886, issued as U.S. Pat. No. 7,478,675,entitled “Extended Settable Compositions Comprising Cement Kiln Dust andAssociated Methods, filed on Feb. 21, 2008, which is acontinuation-in-part of U.S. patent application Ser. No. 11/223,669,issued as U.S. Pat. No. 7,445,669, entitled “Settable CompositionsComprising Cement Kiln Dust and Additive(s),” filed Sep. 9, 2005, theentire disclosures of which are incorporated herein by reference.

BACKGROUND

The present invention relates to cementing operations and, moreparticularly, in certain embodiments, to acid-soluble cementcompositions that comprise cement kiln dust (“CKD”) and associatedmethods of use.

Cement compositions may be used in a variety of subterraneanapplications. For example, in subterranean well construction, a pipestring (e.g., easing, liners, expandable tubulars, etc.) may be run intoa well bore and cemented in place. The process of cementing the pipestring in place is commonly referred to as “primary cementing.” In atypical primary cementing method, a cement composition may be pumped,into an annulus between the walls of the well bore and the exteriorsurface of the pipe string disposed therein. The cement composition mayset in the annular space, thereby forming an annular sheath of hardened,substantially impermeable cement (i.e., a cement sheath) that maysupport and position the pipe string in the well bore and may bond theexterior surface of the pipe string to the subterranean formation. Amongother things, the cement sheath surrounding the pipe string functions toprevent the migration of fluids in the annulus, as well, as protectingthe pipe string from corrosion. Cement compositions also may be used inremedial cementing methods, for example, to seal cracks or holes in pipestrings or cement sheaths, to seal highly permeable formation zones orfractures, to place a cement plug, and the like. Cement compositionsalso may be used in surface applications, for example, constructioncementing.

In some applications, it may be desirable for the cement composition tobe acid soluble. For instance, an acid-soluble cement composition may bedesirable in applications where it is anticipated that the hardenedcement will be removed in subsequent well, bore operations. Oneparticular application includes use of an acid-soluble cementcomposition to plug permeable zones in a formation that may allow the ondesired flow of fluid into, or from, the well bore. For example, thepermeable zones may result in the loss of circulation of fluids, such asa drilling fluid or a cement composition, in the well bore or an ondesired influx of gas or water into the well bore. The permeable zonesinclude, for example, vugs, voids, fractures (natural or otherwiseproduced) and the like. Other applications for acid-soluble cementcompositions include, for example, the formation of annular plugs andisolation of gravel-packed well bore intervals. Examples of acid-solublecement compositions include those comprising Sorel cements and Portlandcements.

SUMMARY

The present invention relates to cementing operations and, moreparticularly, in certain embodiments, to acid-soluble cementcompositions that comprise CKD and associated methods of use.

An embodiment of the present invention provides a method of cementingcomprising: providing an acid-soluble cement composition comprising akiln dust and water; allowing the acid-soluble cement composition to setto form an acid-soluble hardened mass; and contacting the acid-solublehardened mass with an acid.

Another embodiment of the present invention provides a method cementing.The method of cementing may comprise placing an acid-soluble cementcomposition in a subterranean formation. The acid-soluble cementcomposition may comprise cement kiln dust in an amount of 100% by weightof a total amount of cementitious components in the acid-soluble cementcomposition and water. The method further may comprise allowing theacid-soluble cement composition to set to form an acid-soluble hardenedmass. The method further may comprise contacting the acid-solublehardened mass with an acid.

Another embodiment of the present invention provides a method ofcementing. The method may comprise placing an acid-soluble cementcomposition in a subterranean formation. The acid-soluble cementcomposition may comprise cement kiln dust and water, wherein theacid-soluble cement composition is free of any acid-soluble fillers. Themethod further may comprise allowing the acid-soluble cement compositionto set to form an acid-soluble hardened mass. The method further maycomprise contacting the acid-soluble hardened mass with an acid.

The features and advantages of the present invention will be readilyapparent to those skilled in the art. While numerous changes may be madeby those skilled in the art, such changes are within the spirit of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to cementing operations and, moreparticularly, in certain embodiments, to acid-soluble cementcompositions that comprise CKD and associated methods of use. There maybe several potential advantages to the methods and compositions of thepresent invention, only some of which may be alluded to herein. One ofthe many potential advantages of embodiments of the present invention isthat the inclusion of the CKD in the acid-soluble cement compositionsshould reduce the amount of, or potentially eliminate, a higher costadditive, such as Portland or Sorel cement, resulting in a moreeconomical cement composition. Another potential advantage ofembodiments of the present invention is that reduction of the amount ofPortland cement should reduce the carbon footprint of the acid-solublecement compositions.

Embodiments of the acid-soluble cement compositions of the presentinvention, may comprise CKD. Additional embodiments of the acid-solublecement compositions may comprise a hydraulic cement; a componentselected from the group consisting of CKD, a natural pozzolan, and acombination thereof; and water. In an embodiment, the hydraulic cementmay comprise Sorel cement. In another embodiment the cement compositionsmay further comprise an acid-soluble filler. In additional embodiments,the cement compositions may comprise CKD and be tree of any acid-solublefillers. In yet another embodiment, the cement compositions may furthercomprise a source of calcium ions (e.g., hydrated lime). Other optionaladditives may also be included in embodiments of the cement compositionsof the present invention as desired, including, but not limited to, flyash, slag cement, metakaolin, shale, zeolite, combinations thereof, andthe like. Additionally, embodiments of the cement compositions of thepresent invention may be foamed and/or extended as desired by those ofordinary skill in the art.

The acid-soluble cement compositions of the present invention shouldhave a density suitable for a particular application as desired by thoseof ordinary skill in the art, with the benefit of this disclosure. Insome embodiments, the cement compositions of the present invention mayhave a density in the range of from about 8 pounds per gallon (“ppg”) toabout 16 ppg. In other embodiments, the cement compositions may befoamed to a density in the range of from about 8 ppg to about 13 ppg.

Embodiments of the acid-soluble cement compositions of the presentinvention may comprise a hydraulic cement. A variety of hydrauliccements may be utilized in accordance with the present invention,including, but not limited to, those comprising calcium, aluminum,silicon, oxygen, iron, and/or sulfur, which set and harden by reactionwith water. Suitable hydraulic cements include, but are not limited toSorel cements, Portland cements, pozzolana cements, gypsum cements, highalumina content cements, slag cements, silica cements, and combinationsthereof. In certain embodiments, the hydraulic cement may comprise aPortland cement. In some embodiments, the Portland cements that aresuited for use in the present invention are classified as Classes A, C,G, and H cements according to American Petroleum Institute, APISpecification for Materials and Testing for Well Cements, APISpecification 10, Fifth Ed., Jul. 1, 1990. In addition, in someembodiments, cements suitable for use in the present invention may beclassified as ASTM Type I, II, or III As will be discussed in moredetail below, acid-soluble fillers can be used with hydraulic cements(such as Portland cement) that do harden into an acid-soluble mass.

Where present, the hydraulic cement generally may be included in theacid-soluble cement compositions in an amount sufficient to provide thedesired compressive strength, density, and/or cost. In accordance withembodiments, at least a portion of the hydraulic cement and potentiallyeven all of the hydraulic cement may be replaced with CKD and/or anatural pozzolan. In an embodiment, at a least a portion of thehydraulic cement is replaced, with CKD and/or a natural pozzolan. Insome embodiments, the hydraulic cement may be present in the cementcompositions of the present invention in an amount in the range of 0% toabout 99% by weight of cementitious components. As used herein, the term“by weight of cementitious components” refers to the concentration ofthe particular component by weight of a total amount of cementitiouscomponents included in the cement composition. Cementitious componentsinclude those components or combinations of components of the cementcompositions that hydraulically set, or otherwise harden, to developcompressive strength, including, for example, Sorel cement Portlandcement, CKD, fly ash, pumice, slag, lime, shale, and the like. Forexample, the cementitious components may comprise the hydraulic cementand any additional cementitious components that may be present in theacid-soluble cement composition. The hydraulic cement may be present, incertain embodiments, in an amount of about 5%, about 10%, about 15%,about 20%, about 25%, about 30%, about 35%, about 40%, about 45%, about50%, about 55%, about 60%, about 65%, about 70%, about 75%, about 80%,about 90%, or about 95%. In an embodiment, the hydraulic cement may bepresent in an amount in the range of 0% to about 95% by weight ofcementitious components, la another embodiment, the hydraulic cement maybe present in an amount in the range of about 20% to about 95% by weightof cementitious components. In yet another embodiment, the hydrauliccement may be present m an amount in the range of about 50% to about 90%by weight of cementitious components. One of ordinary skill in the art,with the benefit of this disclosure, will recognize the appropriateamount of the hydraulic cement to include for a chosen application.

An example of a suitable hydraulic cement comprises a Sorel cement.Sorel cements typically include magnesia-based cement systems formedfrom a mixture of magnesium oxide and magnesium chloride. However, asused herein, the term “Sorel cement” is intended to encompass any of avariety of metal oxides and soluble salts which together form, ahydraulic cement. In the presence of water, the metal oxide and thesoluble salt forming the Sorel cement should solidify into anacid-soluble mass. Embodiments of the Sorel cements should rapidlydevelop a desirable compressive strength. In accordance with,embodiments, at least a portion of the Sorel cement may be replaced withCKD and/or a natural pozzolan. In an embodiment, at a least a portion ofthe soluble salt is replaced with CKD and/or a natural pozzolan.

In an embodiment, the Sorel cement comprises a metal oxide. In oneparticular embodiment, the Sorel cement comprises an alkaline earthmetal oxide, such as magnesium oxide. A suitable metal oxide isTHERMATEK™ LT additive, available from Halliburton Energy Services, Inc.The metal oxide present in the Sorel cement should have an activitylevel sufficient to provide the desired reactivity. For example, thehigher the activity level, of the metal oxide, the fester the reactionof the metal oxide with the other components of the Sorel cement, toform the hardened mass. The activity level of the metal oxide may varybased on a number of factors. For example, the particle sizedifferential of the metal oxide particles may affect the activity level.A smaller particle size differential, may result in a higher activitylevel due, inter alia, to a greater surface area. Another factor thatmay affect the activity level of the metal oxide is a sintering process.By varying the heat applied during, and time of, the sintering process,metal oxide with varying activity levels may be provided. Metal oxidethat has not been treated, by a sintering process may have a very highactivity level, and thus it may be highly reactive in the Sorel cements.In an embodiment, a relatively more reactive metal oxide may be desired,such as where it may be desired to have a cement composition with arelatively short set time, for example, when desired to rapidly seal offa permeable zone. In an alternative embodiment, a relatively lessreactive metal, oxide may be desired, for example, where a delay may bedesired between, mixing the cement composition and the formation of ahardened mass.

A wide variety of soluble salts are suitable for use in the Sorelcement, including metal chlorides. In one embodiment, the Sorel cementcomprises an alkaline earth metal chloride, such as magnesium chloride.An example of a suitable magnesium chloride is C-TEK additive, availablefrom Halliburton Energy Services, Inc. In an alternative embodiment, theSorel cement comprises magnesium sulfate or ammonium mono or dibasicphosphate.

In an embodiment, the Sorel cement may comprise the metal oxide and thesoluble salt in a metal-oxide-to-soluble-salt ratio of about 3:1 toabout 1:3. In another embodiment, the metal-oxide-to-soluble-salt ratiomay range from about 2:1 to about 1:2. In yet another embodiment, themetal-oxide-to-soluble-salt ratio may range from about 1.5:1 to about1:1.5. One of ordinary skill in the art will recognize the appropriateratio of the metal oxide and soluble salt to include for a particularapplication.

Embodiments of the acid-soluble cement compositions generally maycomprise CKD, which is a material generated in the manufacture ofcement. CKD, as that term is used herein, refers to a partially calcinedkiln feed which is removed from the gas stream and collected, forexample, in a dust collector during the manufacture of cement. Usually,large quantities of CKD are collected in the production of cement thatare commonly disposed of as waste. Disposal of the CKD as waste can addundesirable costs to the manufacture of the cement, as well as theenvironmental concerns associated with its disposal. The chemicalanalysis of CKD from various cement manufactures varies depending on anumber of factors, including the particular kiln feed, the efficienciesof the cement production operation, and the associated, dust collectionsystems. CKD generally may comprise a variety of oxides, such as SiO₂,Al₂O₃, Fe₂O₃, CaO, MgO, SO₃, Na₂O, and K₂O. The term “CKD” is usedherein to mean cement kiln dust made as described above and equivalentforms of cement kiln dust made in other ways.

The CKD generally may exhibit cementitious properties, in that it mayset and harden in the presence of water. In accordance with embodimentsof the present invention, the CKD may be used, among other things, toreplace higher cost cementitious components, such as Portland cementand/or Sorel cement, resulting in more economical cement compositions.In addition, substitution of the CKD for the Portland and/or Sorelcement should result in a cement composition with a reduced carbonfootprint.

The CKD may be included in the acid-soluble cement compositions in anamount sufficient to provide the desired compressive strength, density,cost reduction, and/or reduced carbon footprint. In some embodiments,the CKD may be present in the cement compositions of the presentinvention in an amount in the range of from about 1% to 100% by weightof cementitious components, for example, the CKD may be present in anamount of about 5%, about 10%, about 15%, about 20%, about 25%, about30%, about 35%, about 40%, about 45%, about 50%, about 55%, about 60%,about 65%, about 70%, about 75%, about 80%, about 90%, or about 95%. Inone embodiment, the CKD may be present in an amount in the range of fromabout 5% to about 99% by weight of cementitious components. In anotherembodiment, the CKD may be present in an amount in the range of fromabout 5% to about 80% by weight of cementitious components. In yetanother embodiment, the CKD may be present in an amount in the range offrom about 50% to about 80% by weight of cementitious components. One ofordinary skill in the art, with the benefit of this disclosure, willrecognize the appropriate amount, of CKD to include for a chosenapplication.

While the preceding description describes CKD, the present invention isbroad enough to encompass the use of other partially calcined kiln feedsthat may be present in embodiments of the cement compositions of thepresent invention in an amount in a range of form about 1% to about 100%by weight of cementitious components. For example, embodiments of theacid-soluble cement compositions may comprise lime kiln dust, which, isa material that is generated during the manufacture of lime. The term“lime kiln dust” typically refers to a partially calcined kiln feedwhich can be removed from the gas stream and collected, for example, ina dust collector during the manufacture of lime. The chemical analysisof lime kiln, dust from, various lime manufactures varies depending on anumber of factors, including the particular limestone or dolomiticlimestone feed, the type of kiln, the mode of operation of the kiln, theefficiencies of the lime production operation, and the associated dustcollection systems, lime kiln dust generally may comprise varyingamounts of free lime and free magnesium, lime stone, and/or dolomiticlimestone and a variety of oxides, such as SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO,SO₃, Na₂O, and K₂O, and other components, such as chlorides.

Embodiments of the acid-soluble cement compositions may further comprisea natural pozzolan. Natural pozzolans are generally present on theEarth's surface and set and harden in the presence of hydrated lime andwater. Examples of natural pozzolans include pumicite, diatomaceousearth, volcanic ash, opaline shale, tuff, and combinations thereof.Generally, pumicite is a volcanic rock that exhibits cementitiousproperties, in that it may set and harden in the presence of a source ofcalcium ions and water. Hydrated lime may be used in combination withthe pumicite, for example, to provide sufficient calcium ions for thepumicite to set. The natural pozzolan may be used, among other things,to replace higher cost cementitious components, such as Portland orSorel cement, in embodiments of the sealant compositions, resulting inmore economical sealant compositions. In addition, substitution of thenatural pozzolan for the Portland cement and/or Sorel cement shouldresult in a cement composition with a reduced carbon footprint.

Where present, the natural pozzolan may be included in an amountsufficient to provide the desired compressive strength, density, costreduction and/or reduced carbon footprint for a particular application.In some embodiments, the natural pozzolan may be present in theacid-soluble cement compositions of the present invention in an amountin the range of from about 1% to about 100% by weight of cementitiouscomponents. For example, the natural pozzolan may be present in anamount of about 5%, about 10%, about 15%, about 20%, about 25%, about30%, about 35%, about 40%, about 45%, about 50%, about 55%, about 60%,about 65%, about 70%, about 75%, about 80%, about 90%, or about 95%. Inone embodiment, the natural pozzolan may be present in an amount in therange of from about 5% to about 99% by weight of cementitiouscomponents, hi another embodiment, the natural pozzolan may be presentin an amount in the range of from about 5% to about 80% by weight ofcementitious components. In yet another embodiment, the natural pozzolanmay be present in an amount in the range of from about 10% to about 50%by weight of cementitious components. In yet another embodiment, thenatural pozzolan may be present in an amount in the range of from about25% to about 50% by weight of cementitious components. One of ordinaryskill in the art, with the benefit, of this disclosure, will recognizethe appropriate amount of the natural pozzolan to include for a chosenapplication.

The water that may be used in embodiments of the cement compositions mayinclude, for example, freshwater, saltwater (e.g., water containing oneor more salts dissolved therein), brine (e.g., saturated, saltwaterproduced from subterranean formations), seawater, or combinationsthereof. Generally, the water may be from any source, provided that thewater does not contain an excess of compounds that may undesirablyaffect other components in the cement composition. In some embodiments,the water may be included in an amount, sufficient to form a pumpableslurry. In some embodiments, the water may be included in the cementcompositions of the present invention in an amount in the range of about40% to about 2.00% by weight of cementitious components. In someembodiments, the water may be included in an amount in the range ofabout 40% to about 150% by weight of cementitious components. One ofordinary skill in the art, with the benefit of this disclosure, willrecognize the appropriate amount of water to include for a chosenapplication.

Embodiments of the cement compositions may further comprise a source ofcalcium ions, such as lime. In certain embodiments, the source ofcalcium ions may include hydrated lime. The source of calcium ions maybe included in embodiments of the cement compositions, for example to,form a hydraulic composition with other components of the cementcompositions, such as the pumice, fly ash, slag, and/or shale. Wherepresent, the lime may be included in the cement compositions in anamount sufficient for a particular application. In some embodiments, thelime may be present in an amount in the range of from about 1% to about40% by weight of cementitious components. For example, the lime may bepresent in an amount, of about 5%, about 10%, about 15%, about 20%,about 25%, about 30%, or about 35%. In one embodiment, the lime may bepresent in an amount in the range of from about 5% to about 20% byweight of cementitious components. One of ordinary skill in the art,with the benefit of this disclosure, will recognize the appropriateamount of the lime to include for a chosen application.

Embodiments of the acid-soluble cement compositions may further comprisean acid-soluble filler. The acid-soluble filler may be used, forexample, in compositions that comprise Portland cement with theacid-soluble filler providing an acid-soluble component so that thecompositions can be dissolved and removed. In an embodiment, theacid-soluble filler is present in a cement composition comprising aSorel cement. Examples of suitable acid-soluble filler materials thatare non-reactive with other components in the compositions, includingwithout limitation dolomite, magnesium carbonate, calcium carbonate, andzinc carbonate. Where used, the acid-soluble filler may be present inthe acid-soluble cement composition in an amount of from about 0.1% toabout 300% by weight of the cementitious component. In an embodiment,the acid-soluble filler is present in an amount of from about 50% toabout 400% by weight of the cementitious component. In an embodiment,the acid-soluble filler is present in an amount of from about 100% toabout 300% by weight of the cementitious component. In alternativeembodiments, the acid-soluble cement compositions may be free of theacid-soluble filler in that the acid-soluble cement compositionscomprises the acid-soluble filler in an amount of about 0% by weight ofthe cementitious component. One of ordinary skill in the art, with thebenefit of this disclosure, will recognize the appropriate amount of theacid-soluble filler to include for a chosen application.

Embodiments of the acid-soluble cement compositions may further comprisefly ash. A variety of fly ashes may be suitable, including fly ashclassified as Class C and Class F fly ash according to AmericanPetroleum Institute, API Specification for Materials and Testing forWell Cements, API Specification 10, Fifth Ed., Jul. 1, 1990. Class C flyash comprises both silica and lime so that, when mixed with water, itshould set to form a hardened mass. Class F fly ash generally does notcontain sufficient lime, so an additional source of calcium ions isrequired for the Class F fly ash to form a hydraulic composition. Insome embodiments, lime may be mixed with Class F fly ash in an amount inthe range of about 0.1% to about 25% by weight of the fly ash. In someinstances, the lime may be hydrated lime. Suitable examples of fly ashinclude, but are not limited to, POZMIX® A cement additive, commerciallyavailable from Halliburton Energy Services, Inc., Duncan, Okla.

Where present, the fly ash generally may be included in the acid-solublecement compositions in an amount sufficient to provide the desiredcompressive strength, density, and/or cost, in some embodiments, the flyash may be present in the cement compositions of the present inventionin an amount in the range of about 5% to about 75% by weight ofcementitious components. In some embodiments, the fly ash may be presentin an amount in the range of about 10% to about 60% by weight ofcementitious components. One of ordinary skill in the art, with thebenefit of this disclosure, will recognize the appropriate amount of thefly ash to include for a chosen application.

Embodiments of the acid-soluble cement compositions may further comprisea slag cement. In some embodiments, a slag cement that may be suitablefor use may comprise slag. Slag generally does not contain sufficientbasic material, so slag cement, may further comprise a base to produce ahydraulic composition that may react with water to set to form ahardened mass. Examples of suitable sources of bases include, but arenot limited to, sodium, hydroxide, sodium bicarbonate, sodium carbonate,lime, and combinations thereof.

Where present, the slag cement generally may be included in theacid-soluble cement compositions in an amount sufficient to provide thedesired compressive strength, density, and/or cost. In some embodiments,the slag cement may be present, in the cement, compositions of thepresent invention in an amount in the range of about 0.1% to about 99%by weight of cementitious components. In some embodiments, the slagcement may be present in an amount in the range of about 5% to about 75%by weight of cementitious components. One of ordinary skill in the art,with the benefit of this disclosure, will recognize the appropriateamount of the slag cement to include for a chosen application.

Embodiments of the acid-soluble cement compositions may further comprisemetakaolin. Generally, metakaolin is a white pozzolan that may beprepared, by heating kaolin clay, for example, to temperatures in therange of about 600° C. to about 800° C. In some embodiments, themetakaolin may be present in the cement compositions of the present,invention in an amount in the range of about 5% to about 95% by weightof cementitious components. In some embodiments, the metakaolin may bepresent in an amount in the range of about 10% to about 50% by weight ofcementitious components. One of ordinary skill in the art, with thebenefit of this disclosure, will recognize the appropriate amount of themetakaolin to include for a chosen application.

Embodiments of the acid-soluble cement compositions may further compriseshale. Among other things, shale included in the cement compositions mayreact with excess lime to form a suitable cementing material, forexample, calcium silicate hydrate. A variety of shales may be suitable,including those comprising silicon, aluminum, calcium, and/or magnesium.An example of a suitable shale comprises vitrified shale. Suitableexamples of vitrified shale include, but are not limited to,PRESSUR-SEAL FINE LCM material and PRESSUR-SEAL COARSE LCM materialwhich are commercially available from TXI Energy Services, Inc.,Houston, Tex. Generally, the shale may have any particle sizedistribution as desired for a particular application. In certainembodiments, the shale may have a particle size distribution in therange of about 37 micrometers to about 4,750 micrometers.

Where present, the shale may be included in the acid-soluble cementcompositions of the present invention in an amount sufficient to providethe desired compressive strength, density, and/or cost. In someembodiments, the shale may be present in the cement compositions of thepresent invention in an amount in the range of about 5% to about 75% byweight of cementitious components. In some embodiments, the shale may bepresent in an amount in the range of about 10% to about 35% by weight ofcementitious components. One of ordinary skill in the art, with thebenefit of this disclosure, will recognize the appropriate amount of theshale to include for a chosen application.

Embodiments of the acid-soluble cement compositions may further comprisezeolite. Zeolites generally are porous alumino-silicate minerals thatmay be either a natural or synthetic material. Synthetic zeolites arebased on the same type of structural cell as natural zeolites, and maycomprise aluminosilicate hydrates. As used herein, the term “zeolite”refers to all natural and synthetic forms of zeolite. Examples ofsuitable zeolites are described in more detail in U.S. Pat. No.7,445,669. An example of a suitable source of zeolite is available fromthe C2C Zeolite Corporation of Calgary, Canada. In some embodiments, thezeolite may be present in the cement compositions of the presentinvention, in an amount in the range of about 5% to about 65% by weightof cementitious components. In certain embodiments, the zeolite may bepresent in an amount in the range of about 10% to about 40% by weight ofcementitious components. One of ordinary skill in the art, with thebenefit of this disclosure, will recognize the appropriate amount of thezeolite to include for a chosen application.

Embodiments of the acid-soluble cement compositions may further comprisea set-retarding additive. As used herein, the term “set-retardingadditive” refers to an additive that retards the setting of theacid-soluble cement compositions of the present invention. Examples ofsuitable set-retarding additives include, but are not limited to,ammonium, alkali metals, alkaline earth metals, metal salts ofsulfoalkylated lignins, organic acids (e.g., hydroxycarboxy acids),copolymers that comprise acrylic acid or maleic acid, and combinationsthereof. One example of a suitable sulfoalkylated lignin comprises asulfomethylated lignin. Suitable set-retarding additives are disclosedin more detail in U.S. Pat. No. Re. 31,190, the entire disclosure ofwhich is incorporated herein by reference. Suitable set-retardingadditives are commercially available from Halliburton Energy Services,Inc. under the trademarks HR®-4, HR®-5, HR®-7, HR®-12, HR®-15, HR®-25,HR®-601, SCR™-100, and SCR™-500 retarders. Generally, where used, theset-retarding additive may be included in the cement compositions of thepresent invention in an amount sufficient to provide the desired setretardation. In some embodiments, the set-retarding additive may bepresent in the cement compositions of the present invention an amount inthe range of about 0.1% to about 5% by weight of cementitiouscomponents. One of ordinary skill in the art, with the benefit of thisdisclosure, will, recognize the appropriate amount of the set-retardingadditive to include for a chosen application.

Optionally, other additional additives may be added to the acid-solublecement compositions of the present invention as deemed appropriate byone skilled in the art, with the benefit of this disclosure. Examples ofsuch additives include, but are not limited to, strength-retrogressionadditives, set accelerators, weighting agents, lightweight additives,gas-generating additives, mechanical-property-enhancing additives,lost-circulation, materials, filtration-control additives, dispersants,fluid-loss-control additives, defoaming agents, foaming agents,oil-swellable particles, water-swellable particles, thixotropicadditives, and combinations thereof. Specific examples of these, andother, additives include crystalline silica, amorphous silica, fumedsilica, salts, fibers, hydratable clays, microspheres, rice husk ash,elastomers, elastomeric particles, resins, latex, combinations thereof,and the like. A person having ordinary skill in the art, with thebenefit of this disclosure, will readily be able to determine the typeand amount of additive useful for a particular application and desiredresult.

The components of the acid-soluble cement compositions may be combinedin any order desired to form an acid-soluble cement composition that canbe placed into a subterranean formation. In addition, the components ofthe acid-soluble cement compositions may be combined using any mixingdevice compatible with the composition, including a bulk mixer, forexample. In some embodiments, a dry blend may first be formed by dryblending dry components comprising, for example, CKD and/or hydrauliccement. The dry blend may then be combined with water to form theacid-soluble cement composition. Other suitable techniques may be usedfor preparation of the acid-soluble cement compositions as will beappreciated by those of ordinary skill in the art in accordance withembodiments of the present invention.

As will be appreciated by those of ordinary skill in the art, theacid-soluble cement compositions of the present invention may be used insubterranean operations in accordance with embodiments of the presentinvention. Without limitation, the cement composition may be used toseal off one or more subterranean zones from a well bore; to plug a voidor crack in a conduit disposed in the well bore; to plug a void or crackin a cement, sheath disposed in the well bore; to plug an openingbetween the cement sheath and the conduit; to prevent the loss of fluidfrom the well bore into loss circulation zones such as a void, vug, orfracture; to form an annular plug; to isolate a gravel packed, intervalof the well bore; or combinations thereof. In an embodiment, theacid-soluble cement composition may be used to form an acid-solublebarrier (e.g., a plug a seal, etc) in a subterranean formation. Forexample, the acid-soluble cement composition may be introduced into awell-bore annulus and allowed to set to form an acid-soluble cement,sheath.

An example of a method of the present invention comprises placing anacid-soluble cement composition in a subterranean formation, andallowing the acid-soluble cement composition to set in the formation. Itis intended to be understood that the phrase “placing an acid-solublecement composition in the subterranean formation” encompasses placementof the cement composition in the well bore and/or placement of thecement composition in rock surrounding the well bore with the well borepenetrating the subterranean formation, among others. The cementcomposition should form an acid-soluble hardened mass in thesubterranean formation. The acid-soluble hardened mass can be left inthe subterranean formation permanently or can be removed. Removal of thehardened mass may be desired so that the subterranean formation can beutilized in subsequent hydrocarbon production in accordance withembodiments of the present invention. In an embodiment, removal of thehardened mass includes contacting the hardened mass with an aqueous acidcomposition to at least partially dissolve the hardened mass. In someembodiments, the hardened mass may be completed removed. In otherembodiments, the hardened mass may be partially removed. For example,the aqueous acid composition may contact the hardened mass to formthrough openings in the hardened mass to place the subterraneanformation in communication with the interior of a pipe string, forexample. The aqueous acid composition may include, for example, fromabout 7.5% to about 28% hydrochloric acid by weight of the composition.In an embodiment, the aqueous acid composition includes hydrochloricacid in an amount of about 15% by weight.

Another example of a method of the present invention comprises placingan acid-soluble cement composition in a well-bore annulus (e.g., anannulus between a pipe string disposed in a well bore and a wall of thewell bore); and allowing the acid soluble cement composition to set. Forexample, the acid-soluble cement, composition may set in the well-boreannulus to form an acid-soluble cement sheath. The acid-soluble cementsheath can be left in the subterranean formation permanently or can beremoved. Removal of the hardened mass may be desired so that thesubterranean formation can be utilized in subsequent hydrocarbonproduction in accordance with embodiments of the present invention. Inan embodiment, removal of the hardened mass includes contacting thehardened mass with an aqueous acid composition to at least partiallydissolve the hardened mass. For example, the aqueous acid compositionmay contact the hardened mass to form through openings in the hardenedmass to place the subterranean formation in communication with theinterior of a pipe string, for example. In some embodiment, the aqueousacid composition may be placed into the well bore and allowed to contactthe hardened mass through one or more openings in the pipe string.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit or define, thescope of the invention.

EXAMPLE 1

A series of acid-soluble cement compositions was prepared at roomtemperature and subjected to crush strength and solubility testing. Eachof the samples contained sufficient water to provide the densityprovided in the table below and comprised various quantities of Class HPortland cement, Holcim CKD, and/or calcium carbonate, as indicated inthe table below.

Solubility Testing: For the solubility testing, each sample was pouredinto a 2-inch cube and allowed to cure in a water bath at 150° F. foreither 48 hours (Samples 1-5) or 72 hours (Samples 6-10). After curing,the sample cubes are placed in an 80° F. water bath for at least 30minutes and then weighed to determine an initial weight. Each samplecube was then submerged in 2,000 milliliters of a 15% by weighthydrochloric acid solution in a 3,000 milliliter beaker. The sample cubewas supported in the acid solution above a magnetic stir bar. Themagnetic stir bar was rotated to create a slight vortex on the surfaceof the acid solution. After 30 minutes, the sample cube was removed fromthe acid solution and weighed to determine a final weight. The acidsolubility of each composition was calculated by the following formula:Acid Solubility=(Initial Weight−Final Weight)/Initial Weight×100

Crush Strength Testing: For the crush strength testing, each sample waspoured into a 2-inch cube, allowed to cure in a water bath at 150° C.for 48 hours (Samples 1-5) or 72 hours (Samples 6-10), and then crushed.The crush strengths were determined using a Tinius Olson tester inaccordance with API Specification 10.

The results of the tests are set forth in the table below. In thefollowing table, percent by weight is based on the weight of the cementand the CKD in the samples.

TABLE 1 Crush Strength Tests: Cement, CKD, and CaCO₃ PortlandDissolution Crush Cement CKD CaCO₃ Time in Acid Strength Density (% by(% by (% by 15% HCl Solubility (psi) Sample (ppg) wt) wt) wt) (min) (%)48 Hr 72 Hr 1 15 75 25 100 30 82.72 736 — 2 15 50 50 100 30 99.01 523 —3 15 25 75 100 30 99.88 353 — 4 15 0 100 100 30 99.25 67.2 — 5 15 100 0100 30 66.46 1004 — 6 14.5 75 25 300 30 90.97 — 152 7 14.5 50 50 300 3092.61 — 108 8 14.5 25 75 300 30 93.55 — 61 9 14.5 0 100 300 30 99.38 —20 10 14.5 100 0 300 30 93.45 — 188

Example 1 thus indicates that acid-soluble cement compositionscontaining from 25% to 100% CKD by weight, from 0% to 75% Portlandcement by weight, and from 100% to 300% calcium carbonate by weight mayhave properties suitable for use in acid-soluble operations.

EXAMPLE 2

An additional series of acid-soluble cement compositions was preparedand subjected to thickening time, force resistance, and theologicaltests. Each of the samples contained sufficient water to provide thedensity provided in the table below and comprised various quantities ofClass H Portland cement, Holcim CKD, calcium carbonate, a dispersant(CFR™-3 cement friction reducer), and/or a set-retarding additive, asindicated in the table below. In the following tables, percent by weightis based on the weight of the cement and the CKD in the samples.

The samples were prepared at room temperature with thickening time testsconducted at 140° F. on a portion of each composition in accordance withAPI Specification 10. The crush strength of Sample 12 was determined bypouring the sample into a 2-inch cube, allowing it to cure in a waterbath at 160° F. for 72 hours, and then crashing the cured cube. Thecrush strength was determined using a Tinius Olson tester in accordancewith API Specification 10. The results of the thickening time and forceresistance tests are provided in the table below.

TABLE 2 Thickening Time Tests: Cement, CKD, and CaCO₃ Portland CKDThickening 72-Hr Cement (% CaCO₃ Retarder Time Crush Density (% by by (%by Dispersant (% by to 70 bc Strength Sample (ppg) wt) wt) wt) (% by wt)wt) (hr:min) (psi) 11 15 50 50 100 — 0.25% 2:31 — HR ®-5 12 15 50 50 100— 0.5% 3:27 545 HR ®-5 13 16 75 25 100 0.5 0.5% 1:41 — SCR-5 ™ 14 16 7525 100 — 1% 8:42 — HR ®-12

For the theological tests, additional portions of the acid-solublecement compositions were conditioned in an atmospheric consistometer tothe test temperature. After conditioning, the theology of thecompositions was determined using a Fann Model 35 viscometer at thetemperature indicated in the table below using a bob and sleeve andspring #1 in accordance with the procedure set forth in APISpecification 10. The results of the rheological tests are set forth inthe table below. In the following table, percent by weight is based onthe weight of the cement and the CKD in the samples.

TABLE 3 Rheological Tests: Cement, CKD, and CaCO₃ Portland CKD Cement (%CaCO₃ Retarder Density (% by by (% by Dispersant (% by Temp. RotationsPer Minute Sample (ppg) wt) wt) wt) (% by wt) wt) (° F.) 600 300 200 10060 30 6 3 12 15 50 50 100 — 0.5% 80 76 41 30 18 14 9 6 5 HR ®-5 140 4827 21 15 12 9 8 7 13 16 75 25 100 0.5 0.5% 80 167 79 54 27 16 8 2 2SCR-5 ™ 140 52 21 12 6 4 2 1 1 14 16 75 25 100 — 1% 80 207 115 80 47 3322 12 10 HR ®-12 140 98 47 32 18 12 8 4 4

Example 2 thus indicates that acid-soluble cement compositionscontaining from 25% to 50% CKD by weight, from 50% to 75% Portlandcement by weight, and 100% calcium carbonate by weight may haveproperties suitable for use in acid-soluble operations.

EXAMPLE 3

An additional acid-soluble cement composition was prepared to determineforce resistance properties of compositions comprising pumicite. Thecomposition contained sufficient water to provide the density providedin the table below and comprised Class H Portland cement, 200-meshpumicite, calcium carbonate, a set-retarding additive (HR®-5 retarder),and hydrated lime, as indicated in the table below. For the acidsolubility testing, the composition was poured into a 2-inch cube andcured at 180° F. for 24 hours. The acid solubility of the compositionwas then determined by submerging the cured cube in a 15% by weighthydrochloric acid solution in accordance with the procedure describedabove in Example 1. For the crush strength testing, the composition waspoured into a 2-inch cube, allowed to cure in a water bath for 24 hoursat 180° F., and then crushed. The 24-hour crush strength was determinedusing a Tinius Olson tester in accordance with API Specification 10. Theresults of the tests are set forth in the table below. In the followingtable, percent by weight is based on the weight of the cement and theCKD in the samples.

TABLE 4 Crush Strength Tests: Cement, Pumicite, and CaCO₃ PortlandDissolution 24-Hr Cement Pumicite CaCO₃ Retarder Hydrated Time in AcidCrush Density (% by (% by (% by (% by Lime (% 15% HCl SolubilityStrength Sample (ppg) wt) wt) wt) wt) by wt) (min) (%) (psi) 15 15 50 50100 0.5% 5 30 98.62 1400 HR ®-5

Example 3 thus indicates that acid-soluble cement compositionscontaining Portland cement, pumicite, and calcium carbonate may haveproperties suitable for use in acid-soluble operations.

EXAMPLE 4

An additional series of acid-soluble cement compositions was prepared atroom temperature to determine force resistance properties ofcompositions comprising Sorel cement (e.g., a mixture of magnesiumchloride and magnesium oxide), CKD, and/or pumicite. Each, of thesamples contained water, magnesium chloride (C-TEK), magnesium oxide(THERMATEK™ LT additive), Holcim CKD, 200-mesh pumicite, and/or hydratedlime, as indicated in the table below. The crush strength of thecompositions was determined by pouring each composition into a 2-inchcube, allowing the cube to cure in a water bath at 140° F. for either 24or 48 hours, and then crushing the cured cube. The crush strengths weredetermined using a Tinius Olson tester in accordance with APISpecification 10. The results of the tests are set forth in the tablebelow.

TABLE 5 Crush Strength Test: Sorel Cement, CKD, and/or Pumicite CrushStrength Water MgCl2 MgO CKD Pumicite Hydrated (psi) Sample (g) (g) (g)(g) (g) Lime (g) 24 Hr 72 Hr 16 200 300 300 — — — 3460 — 17 200 300 28515 — — — 2430 18 200 300 270 30 — — — 2280 19 200 300 225 75 — — 1116 —20 200 300 225 12.5 12.5 10 — 1822 21 200 300 300 75 — — 1864 — 22 200300 285 — 15 — 3080 — 23 200 300 270 — 30 — 2790 — 24 200 300 225 — 75 —2360 — 25 200 300 225 — 75 7.5 2360 —

Example 4 thus indicates that acid-soluble cement compositionscontaining Sorel cement, cement kiln, dust, and/or pumicite may haveproperties suitable for use in acid-soluble operations.

EXAMPLE 5

An additional series of acid-soluble cement compositions was prepared atroom temperature to determine force resistance properties of lightweightcompositions comprising Sorel cement (e.g., a mixture of magnesiumchloride and magnesium oxide) and CKD. Each of the samples containedwater, magnesium chloride (C-TEK additive), magnesium oxide (THERMATEK™LT additive), Holcim CKD, a set-retarding additive (R-TEK inhibitor),and glass bubbles (HGS 2000 glass bubbles), as indicated in the tablebelow. he crush strength of the compositions was determined by pouringeach composition into a 2-inch cube, allowing the cube to cure in awater bath at 140° F. for 24 hours, and then crushing the cured cube.The crush strengths were determined using a Tinius Olson tester inaccordance with API Specification 10. The results of the tests are setforth in the table below.

TABLE 6 Crush Strength Tests: Sorel Cement and CKD 24-Hr Glass CrushDensity Water MgCl2 MgO CKD Retarder Bubbles Strength Sample (ppg) (g)(g) (g) (g) (g) (g) (psi) 26 11.23 200 300 300 — 18 50 923 27 10.84 200300 225 75 18 50 663

Example 5 thus indicates that acid-soluble cement compositions having alightweight and containing Sorel cement and cement kiln dust may haveproperties suitable for use in acid-soluble operations.

EXAMPLE 6

An additional series of acid-soluble cement compositions was prepared atroom temperature and subjected to thickening time tests at 140° F. inaccordance with API Specification 10. Each of the samples containedwater, magnesium chloride (C-TEK additive), magnesium oxide (THERMATEK™LT additive), Holcim CKD, and a retarder (R-TEK inhibitor) as indicatedin the table below. The results of the tests are set forth in the tablebelow.

TABLE 7 Thickening Time Tests: Sorel Cement and CKD Thickening TimeWater MgCl2 MgO CKD Retarder to 70 bc Sample (g) (g) (g) (g) (g)(hr:min) 28 200 300 225 75 5 00:36  29 200 300 225 75 9 1:13 30 200 300225 75 18 1:11

Example 6 thus indicates that acid-soluble cement compositionscontaining Sorel cement and cement kiln dust may have propertiessuitable for use in acid-soluble operations.

EXAMPLE 7

An additional acid-soluble cement composition was prepared at roomtemperature and subjected to crush strength and solubility testing. Thissample was prepared to test the solubility of an acid-soluble cementcomposition comprising CKD and free of any acid-soluble fillers. Thesample comprised Holcim CKD (25% bwob), Texas Lehigh Class H Portlandcement (25% bwob), fly ash (POZMIX® A cement additive, 25% bwob),bentonite (2.5% bwob), a set-retarding additive (HR®-800 retarder, 0.4%bwob), a fluid-loss-control additive (HALAD®-447, 0.25% bwob), afree-water-control additive (WG-17 EXP free-water control agent, 0.2%bwob), and fresh water (6.2 gal/sk). The abbreviation “% bwob” indicatesthe percent of the component by weight of a cement blend comprising theCKD, Portland cement, and fly ash. The abbreviation “gal/sk” indicatesgallons per 89.5-pound sack of the cement blend. The sample had adensity of 14 pounds per gallon.

Crush Strength Testing: For the crush strength, testing, a portion ofthe sample was poured into a 2-inch cube and allowed to cure in a waterbath at 140° F. for 7 days. After curing, the sample cubes were placedin an 80° F. water bath for at least 30 minutes and then crushed. Thecrash strengths were determined using a Tinius Olson tester inaccordance with API Specification 10. The determined crush strength was2,200 psi.

Solubility Testing: For the solubility testing, a portion of the samplewas poured into a 2-inch cube and allowed to cure in a water bath at140° F. for 48 hours. After curing, the sample cubes were placed in an80° F. water bath for at least 30 minutes and then weighed to determinean initial weight. Each sample cube was then submerged in 2,000milliliters of a 15% by weight hydrochloric acid solution in a 3,000milliliter beaker at ambient conditions. The sample cube was supportedin the acid solution above a magnetic stir bar. The magnetic stir barwas rotated, to create a slight vortex on the surface of the acidsolution. At specified intervals, the sample cube was removed from theacid solution and weighed to determine an interval weight. Weight lossof the cube was determined by subtracting the Interval weight from theinitial weight. The sample cube was then returned to the acid solution.The acid solubility of each composition was calculated by the followingformula:Acid Solubility=Weight Loss/Initial Weight×100After 2 hours, the testing was completed. The results of the solubilitytesting are set forth in the table below.

TABLE 8 Acid-Solubility Tests: 25% CKD, 50% Cement, and 25% Fly Ash in15% HCL Interval Acid Interval Time Weight Weight Loss Solubility(hr:min) (gm) (gm) (%) 0:00 213.76 0 0 0:05 189.44 24.32 11.4 0:10166.04 47.72 22.3 0:20 107.76 106.0 49.6 0:30 82.64 131.12 61.33 0:4560.57 153.19 71.66 1:00 40.69 173.07 80.96 1:15 27.24 186.52 87.25 1:3015.92 197.84 92.6 2:00 7.21 206.55 96.6

The solubility testing was repeated using a 7.5% by weight hydrochloricacid solution. The results of this test are set forth below.

TABLE 9 Acid-Solubility Tests: 25% CKD, 50% Cement, and 25% Fly Ash in7.5% HCL Interval Acid Interval Time Weight Weight Loss Solubility(hr:min) (gm) (gm) (%) 0:00 202.73 0 0 0:05 185.08 17.65 8.71 0:10172.11 30.62 15.10 0:20 147.54 55.19 27.22 0:30 115.68 87.05 42.93 0:4592.85 109.88 54.20 1:00 85.65 117.08 57.75 1:15 79.86 122.87 60.60 1:3075.66 127.07 62.68 2:00 71.03 131.70 64.96

Example 7 thus indicates that acid-soluble cement compositionscontaining CKD and free of an additional acid-soluble filler may havesolubility properties suitable for use in acid-soluble operations.

EXAMPLE 8

An additional acid-soluble cement composition was prepared at roomtemperature and subjected to crush strength and solubility testing. Thissample was prepared to further test the solubility of an acid-solublecement composition comprising CKD and free of any acid-soluble fillers.The sample comprised Holcim CKD (100% bwob), calcium chloride (3% bwob),and fresh water (6.67 gal/sk). The abbreviation “% bwob” indicates thepercent of the component by weight of a cement blend consisting of theCKD. The sample had a density of 13 pounds per gallon.

Crush Strength Testing: For the crush strength testing, a portion of thesample was poured into a 2-inch×4-inch cylinder and allowed to cure in awater bath at 170° F. for 24 hours. After curing, the sample cubes wereplaced in an 80° F. water bath for at least 30 minutes and then crashed.The crush strengths were determined using a Tinius Olson tester inaccordance with API Specification 10. The determined crash strength was345 psi.

Solubility Testing: For the solubility testing, a portion of the samplewas poured into a 2-inch×4-inch cylinder and allowed to cure in a waterbath at 140° F. for 24 hours. After curing, the sample cylinders wereplaced in an 80° F. water bath for at least 30 minutes and then weighed,to determine an initial weight. Each sample cylinder was then submergedin 2,000 milliliters of a 7.5% by weight, hydrochloric acid solution at140° F. in a 3,000 milliliter beaker. The sample cylinder was supportedin the acid solution above a magnetic stir bar. The magnetic stir barwas rotated to create a slight vortex on the surface of the acidsolution. The sample cylinder was observed, and the time for completedissolution of the sample cylinder was recorded. If not completelydissolved, the sample cylinder was removed from the acid solution after2 hours and weighed to determine a final, weight. The acid solubilitywas then was calculated by the following formula:Acid Solubility=(Initial Weight−Final Weight)/Initial Weight×100

The solubility testing was repeated using a 7.5% by weight hydrochloricacid solution and a 15% by weight hydrochloric acid solution. Theresults of the solubility testing are set forth in the table below.

TABLE 10 Acid-Solubility Tests: 100% CKD Dissolution Time AcidSolubility HCl Solution (hr:min) (%) 3.0% 2:00 8 7.5% 1:45 100  15% 0:12100

Example 8 thus indicates that acid-soluble cement compositionscontaining CKD and free of an additional acid-soluble filler may havesolubility properties suitable for use in acid-soluble operations.

It should be understood that the compositions and methods are describedin terms of “comprising,” “containing,” or “including” variouscomponents or steps, the compositions and methods can also “consistessentially of” or “consist of” the various components and steps.Moreover, the indefinite articles “a” or “an,” as used in the claims,are defined herein to mean one or more than one of the element that itintroduces.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as, rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit, is disclosed, any number and anyincluded range falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed, herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even, if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Although individual, embodiments arediscussed, the invention covers all combinations of all thoseembodiments. Furthermore, no limitations are intended to the details ofconstruction or design herein shown, other than as described in theclaims below. Also, the terms in the claims have their plain, ordinarymeaning unless otherwise explicitly and clearly defined by the patentee.It is therefore evident that the particular illustrative embodimentsdisclosed above may be altered or modified and all such variations areconsidered within, the scope and spirit of the present invention. Ifthere is any conflict in the usages of a word or term in thisspecification and one or more patent(s) or other documents that may beincorporated herein by reference, the definitions that are consistentwith this specification should be adopted.

What is claimed is:
 1. A method of cementing comprising: providing anacid-soluble cement composition comprising a kiln dust and water,wherein the acid-soluble cement composition is free of any additionalcementitious components other than the kiln dust; allowing theacid-soluble cement composition to set to form an acid-soluble hardenedmass; and contacting the acid-soluble hardened mass with an acid.
 2. Themethod of claim 1 wherein the acid-soluble cement composition has adensity of about 8 pounds per gallon to about 16 pounds per gallon. 3.The method of claim 1 wherein the kiln dust comprises cement kiln dust.4. The method of claim 1 wherein the kiln dust comprises lime kiln dust.5. The method of claim 1 wherein the kiln dust comprises cement kilndust present in an amount in a range of from about 1% to 100% by weightof a total amount of cementitious components present in the acid-solublecement composition.
 6. The method of claim 1 wherein the kiln dustcomprises cement kiln dust present in an amount in a range of from about50% to about 100% by weight of a total amount of cementitious componentspresent in the acid-soluble cement composition.
 7. The method of claim 1wherein the acid-soluble cement composition is free of an acid-solublefiller.
 8. The method of claim 1 wherein the acid-soluble cementcomposition is free of an acid-soluble filler selected from the groupconsisting of dolomite, magnesium carbonate, calcium carbonate, zinccarbonate, and any combination thereof.
 9. The method of claim 1 whereinthe acid-soluble cement composition further comprises an additiveselected from the group consisting of metakaolin, crystalline silica,amorphous silica, fumed silica, salt, fiber, hydratable clay,microsphere, rice husk ash, an elastomer, an elastomeric particle, aresin, a latex, and any combination thereof.
 10. The method of claim 1wherein the acid-soluble cement composition further comprises anadditive selected from the group consisting of a set-retarding additive,a strength-retrogression additive, a set accelerator, a weighting agent,a lightweight additive, a gas-generating additive, amechanical-property-enhancing additive, a lost-circulation material, afiltration-control additive, a dispersant, a fluid-loss-controladditive, a defoaming agent, a foaming agent, an oil-swellable particle,a water-swellable particle, a thixotropic additive, and any combinationthereof.
 11. The method of claim 1 wherein contacting the acid-solublehardened mass with an acid comprises contacting the acid-solublehardened mass with an aqueous acid composition, wherein the aqueous acidcomposition comprises hydrochloric acid present in the aqueous acidcomposition in an amount of about 7.5% to about 28% by weight of theaqueous acid composition.
 12. The method of claim 1 further comprising:placing the acid-soluble cement composition into a subterraneanformation.
 13. The method of claim 12 wherein the acid-soluble cementcomposition is allowed to set in a well-bore annulus in the subterraneanformation, wherein the acid contacts the acid-soluble hardened massthrough one or more openings in a pipe string disposed in thesubterranean formation.
 14. The method of claim 1 wherein theacid-soluble cement composition further comprises an acid-solublefiller.
 15. A method of cementing comprising: placing an acid-solublecement composition in a subterranean formation, the acid-soluble cementcomposition comprising: cement kiln dust in an amount of 100% by weightof a total amount of cementitious components in the acid-soluble cementcomposition; and water; allowing the acid-soluble cement composition toset to form an acid-soluble hardened mass; and contacting theacid-soluble hardened mass with an acid.
 16. The method of claim 15wherein the acid-soluble cement composition is free of an acid-solublefiller.
 17. The method of claim 15 wherein the acid-soluble cementcomposition is free of an acid-soluble filler selected from the groupconsisting of dolomite, magnesium carbonate, calcium carbonate, zinccarbonate, and any combination thereof.
 18. The method of claim 15wherein the acid-soluble cement composition further comprises anadditive selected from the group consisting of a set-retarding additive,a strength-retrogression additive, a set accelerator, a weighting agent,a lightweight additive, a gas-generating additive, amechanical-property-enhancing additive, a lost-circulation material, afiltration-control additive, a dispersant, a fluid-loss-controladditive, a defoaming agent, a foaming agent, an oil-swellable particle,a water-swellable particle, a thixotropic additive, and any combinationthereof.
 19. The method of claim 15 wherein contacting the acid-solublehardened mass with an acid comprises contacting the acid-solublehardened mass with an aqueous acid composition, wherein the aqueous acidcomposition comprises hydrochloric acid present in the aqueous acidcomposition in an amount of about 7.5% to about 28% by weight of theaqueous acid composition.
 20. The method of claim 15 wherein the placingthe acid-soluble composition comprises placing the acid-solublecomposition in a well-bore annulus between a pipe string disposed in thesubterranean formation and a wall of a well bore.
 21. The method ofclaim 15 wherein the acid-soluble cement composition is allowed to setin a well-bore annulus in the subterranean formation, wherein the acidcontacts the acid-soluble hardened mass through one or more openings ina pipe string disposed in the subterranean formation.
 22. The method ofclaim 15 wherein the acid-soluble cement composition further comprisesan acid-soluble filler.
 23. A method of cementing comprising: placing anacid-soluble cement composition in a subterranean formation, theacid-soluble cement composition comprising cement kiln dust and water,wherein the acid-soluble cement composition is free of any acid-solublefillers; allowing the acid-soluble cement composition to set to form anacid-soluble hardened mass; and contacting the acid-soluble hardenedmass with an acid.
 24. The method of claim 23 wherein the acid-solublecement composition further comprises a hydraulic cement selected fromthe group consisting of a Portland cement, a pozzolana cement, a gypsumcement, a high alumina content cement, a slag cement, a silica cement,and any combination thereof.
 25. The method of claim 23 wherein thecement kiln dust is present in an amount in a range of from about 1% to100% by weight of a total amount of cementitious components present inthe acid-soluble cement composition.
 26. The method of claim 23 whereinthe cement kiln dust is present in an amount of about 100% by weight ofa total amount of cementitious components present in the acid-solublecement composition.
 27. The method of claim 23 wherein the acid-solublecement composition is free of any additional cementitious componentsother than the cement kiln dust.
 28. The method of claim 23 wherein theacid-soluble cement composition further comprises an additive selectedfrom the group consisting of a fly ash, a slag cement, metakaolin,shale, zeolite, crystalline silica, amorphous silica, fumed silica,salt, fiber, hydratable clay, microsphere, rice husk ash, an elastomer,an elastomeric particle, a resin, a latex, and any combination thereof.29. The method of claim 23 wherein the acid-soluble cement compositionfurther comprises an additive selected from the group consisting of aset-retarding additive, a strength-retrogression additive, a setaccelerator, a weighting agent, a lightweight additive, a gas-generatingadditive, a mechanical-property-enhancing additive, a lost-circulationmaterial, a filtration-control additive, a dispersant, afluid-loss-control additive, a defoaming agent, a foaming agent, anoil-swellable particle, a water-swellable particle, a thixotropicadditive, and any combination thereof.
 30. The method of claim 23wherein contacting the acid-soluble hardened mass with an acid comprisescontacting the acid-soluble hardened mass with an aqueous acidcomposition, wherein the aqueous acid composition comprises hydrochloricacid present in the aqueous acid composition in an amount of about 7.5%to about 28% by weight of the aqueous acid composition.
 31. The methodof claim 23 wherein the acid-soluble cement composition is allowed toset in a well-bore annulus in the subterranean formation, wherein theacid contacts the acid-soluble hardened mass through one or moreopenings in a pipe string disposed in the subterranean formation.